SAN FRANCISCO (03/28/2000) - Though image-editing programs such as Adobe Systems Inc.'s Photoshop give you extensive control over images, they can also encourage you to develop some bad habits when scanning originals. Because it's so easy to repair and edit an image, many people give short shrift to the scanning process. However, a little effort pre-Photoshop can not only yield better final images but also save the time you might otherwise spend correcting scans.
Preparing to Scan
First, choose the best material to scan. If you're planning on scanning a print but your scanner has a transparency attachment, consider scanning the original negative instead. Film negatives and slides contain much more color information than a photographic print does, affording you more latitude for color adjustments and edits. If your scanner does a better job with reflective material than with transparencies, or if the print was manipulated or corrected in the printing process, then go with the print.
Photoshop's Dust And Scratches filter does an excellent job of removing small bits of dust, fluff, and hair from an image. An even more effective solution is to clean your scanner's platen. A quick once-over with a soft cloth, paper towel, or sheet of newspaper might be all it takes to remove tiny bits of dust that can be visible in a high-resolution scan. For fingerprints, any type of glass cleaner will work, but check the scanner manual first to be sure that the platen is cleanser-safe.
Though you might be in the habit of laying an original anywhere on the scanner's platen, its sweet spots can provide sharper focus and, therefore, better detail and truer color. Scanners with particularly large platens-legal- or tabloid-size, for example-are more prone to differences in focus quality across their scanning surface. Transparency attachments on flatbed scanners are particularly susceptible to changes in quality from point to point. (Of course, when an image fills the entire platen, sweet spots are irrelevant.) To test for sweet spots, select a high-quality image with good contrast, fine detail, and bright colors. Scan it several times, with the image on a different part of the platen each time. Open all the scans in your image editor and arrange the windows so you can simultaneously view the same section of each scan. Color reproduction should be the same in each scan. If you're using Photoshop, grab the eyedropper tool and check the color values of the same areas of each image. Are they the same? If not, decide which you like best.
Next, zoom in to a high magnification on each image and look for sharpness and focusing problems. In addition to outright blurriness, check for halos of red, green, or blue that may indicate a slight focus problem.
When one image looks better, repeat the test with a different image to make sure you have indeed found a sweet spot.
If you're scanning a transparency, be sure to place the slide or negative in the scanner correctly. Most scanners require the emulsion side (the side of the slide that's less shiny) to be facedown, but check your documentation to be sure.
Before you hit that scan button, you need to define some settings in your software. Most scanners include some type of Photoshop-compatible Acquire plug-in that lets you scan directly into an image-editing application. If your scanner includes such a plug-in, it's a good idea to use it rather than a dedicated scanning application, simply because you won't have to switch between two programs. An Acquire plug-in may even prevent you from accidentally compressing an image as you move it from the scanning application to the image editor.
First, set your scanner to scan in color. In fact, once you've made the setting, leave it there, even when you scan a black-and-white original. Some scanners don't record as much information when set to black-and-white, so you're better off scanning in color and later converting the image to gray-scale. Photoshop does a much better job of converting a color image to black-and-white than your scanner does.
Selecting a resolution is the most important decision you'll make when configuring your scanner. Scan too low, and you'll end up with blurry details and dull color. Scan too high, and you'll not only spend more time scanning and printing but may also have less detail in your final image.
When scanning a transparency, select negative or positive. Don't count on your image editor to invert the image. The scanner needs to know that a negative is a negative.
Many slide scanners include profiles for different types of film; some even allow you to create new profiles. A profile is a description of a film type's color characteristics; scanners use the description to adjust their settings.
Because different film types can have widely varying color gamuts, it's worth taking the time to choose the right profile. Check your scanner documentation for details.
Before you scan a slide or negative using a flatbed scanner's transparency attachment, determine whether your scanner software provides focusing controls.
Most drivers include automatic focusing, and though it takes longer to make the scanner check its focus before scanning, the extra time is worth it. See your manual for details.
You should also turn off any sharpening options. Sharpening is a necessary step, but you don't want the scanner to do it. For optimum quality, it's best to perform sharpening after you've made all other adjustments and edits.
Using Those Extra Bits
Each pixel that makes up a full-color digital image is composed of 24 bits of color data. So why are there 30-bit and 48-bit scanners? The additional bits may seem like marketing hype, but they can actually provide a more accurate image-if you use the scanning software properly.
You can take advantage of those extra bits by using the scanning software, whether it's stand-alone or a Photoshop-compatible plug-in. Like Photoshop, the scanning software has controls for adjusting color, contrast, and levels.
However, your computer is limited to 24 bits of color, while scanners can often process color at greater bit depths. When you use the scanner software's color controls, it makes adjustments using its larger color space; it then downsamples the images to 24-bit color. Why bother with the scanner software when the images end up at 24 bits? Because those extra bits make possible more-precise calculations and finer color-correction control.
As in Photoshop, when your scanner software offers a choice of adjustment types-levels versus simple brightness and contrast adjustments-always use the levels controls. With the less precise brightness and contrast sliders, you may inadvertently change an image's whites and blacks. Levels controls let you adjust contrast while preserving white and black points.
Performing good color correction takes practice, but some general advice can help-whether you're using scanner software or a stand-alone image editor:
-- An image has three basic color categories: highlights, shadows, and midtones. When you see that color is off in an image, determine which of these categories suffers the most. For example, a typical portrait is mostly midtones, with dark shadows spread here and there and highlights on the hair, nose, or glasses. If the portrait has a green cast, correct midtones first.
-- Once you've corrected the central problem, touch up problems in other categories. In the previous example, after you'd removed the greens from the midtones, you would work on the highlights and then on the shadows.
-- Be sure to check the balance of all three categories-adjusting one range of colors sometimes affects the others.
One caveat: If you're preparing images for the Web or a device with a limited gamut, this kind of color control can be overkill.
Saving Time, Saving Files
So you've meticulously tweaked Levels and Curves adjustments and maniacally calculated just the right resolution. Now it's time to preserve those settings.
If you shot a number of pictures in the same location, there's a good chance that they'll need roughly the same color correction and adjustments. If your scanner software can't save settings as a file, you should at least write them down. If you changed locations, film stock, or lighting from one picture to the next, you'll probably have to start from scratch and build new settings.
After you scan an image, save it as a TIFF or PSD file, no matter how you plan to use it. Though the file may be bound for the Web, Web-ready JPEG and GIF files are lossy formats-that is, they degrade image quality-so you shouldn't convert to either GIF or JPEG until you perform all edits. And don't throw out your original scan after the conversion. If you make changes later, perform the edits in the original and then convert to JPEG. Resaving JPEG files removes even more data.
Scanning can introduce some annoying artifacts. If the original is a halftone image from a book or magazine, there's a good chance that moire patterns have crept in. An image editor's Despeckle filter should remove them in one or two passes. Your scanning software may include descreen functions. Experiment with these to see whether they remove the moire without oversoftening the image.
Some scanners produce images with distinct color casts. You can usually remove the cast using a simple Curves adjustment in an image-editing program. If your scanner routinely produces the same color cast, consider building an Action in Photoshop that performs the appropriate adjustment.
Save sharpening for after you resize the image and convert it from RGB to gray-scale. Be sure to use an Unsharp Mask filter rather than a raw Sharpen filter. Unsharp masking produces images with smoother contrast and without oversharpened halos.
It's easy to get crooked scans from flatbed scanners. Fortunately, Photoshop's Measure tool provides a simple way to straighten an image. In your image, find a line that should be horizontal. Select the Measure tool, and click on both ends of that line. Then select Rotate Canvas: Arbitrary from the Image menu.
The resulting dialog box automatically displays the amount of rotation required to straighten your image. Click on OK, and the image is straightened.
Not a Toaster
With scanners as inexpensive as household appliances, it's tempting to treat them the same way. However, scanning isn't yet as routine as making toast. Be mindful when you scan, and you can get better results from a low-end scanner than others could with a more expensive device.
BEN LONG is a freelance writer and illustrator in San Francisco, California.
What to Do when a Scan Is Too Blue
Some scanners regularly produce images with the same incorrect color cast. The easiest way to solve this problem is to alter the image in an image-editing application, such as Adobe Photoshop, after scanning.
You Say You Want a Resolution
Choosing a resolution is not as simple as it seems. The first thing to understand is that there isn't a 1:1 relationship between a dot on your screen and a dot on your printer. So though your printer may boast 1,440 dots per inch (dpi), you don't need anywhere near that resolution in your scan.
Most printers-whether ink-jet, dye-sub, laser, or offset-have only four colors: cyan, magenta, yellow, and black. Four-color printers lay down patterns of dots that we perceive as other colors when we view them from a distance . By contrast, each pixel on your computer screen can be any one of 16 million different colors (assuming your monitor is set to display that amount).
So your printer might reproduce one dark-blue pixel on the screen by creating a pattern of dozens of cyan, magenta, and yellow dots. High-resolution printers can use even more dots, but they still represent that one pixel on your computer screen. The higher printer resolution just means you're less likely to see the individual dots making up the patterns that represent colors.
If the resolution of your file is too low, the printer doesn't have enough data to accurately print detail and color. But if your image resolution is too high, the computer must throw out data, or downsample, before it sends the image to the printer. Downsampling can result in a loss of sharpness and detail, so you should control it when possible. Select a resolution that will provide enough data to create a good image but not so much that the computer will average away detail upon printing.
If your image's destination is an offset press, you need to know the frequency of the halftone screen the printer will use before you can choose the best image resolution. This halftone frequency is usually measured in lines per inch (lpi). Magazines and most good-quality commercial print jobs are usually printed with a halftone frequency of 133 lpi. If your image's final destination is newsprint or photocopies, the printer will probably use a halftone frequency of around 85 lpi.
Many people recommend scanning at a resolution that's twice your final line screen. That means 266 dpi for an image that's destined for a 133-lpi print job. We think it's better to go even lower-around 1.25 to 1.5 times the line screen-to ensure your image doesn't downsample so much that it loses sharpness.
If your image is destined for a desktop ink-jet printer, 200 dpi is usually all you'll ever need. This resolution also works well for laser printers, though you can often go as low as 150 dpi with them.
If your image is headed for a large-format printer, rules for choosing a resolution are mostly the same as those for desk-top and commercial printers.
Because you're printing something that's, say, 3 feet wide and 5 feet long, you may think you need a superhigh resolution, but you don't.
Large-format-printer drivers include special algorithms for scaling images up, and most do wonders with relatively low-res images. We've successfully printed 8-by-10-inch 300-dpi images at massive sizes with excellent results. Consult your service bureau for the nitty-gritty.
Scanners that can capture 600 or 1,200 dpi give you the ability to enlarge an image. Say you've got an image that's 2 inches square and you want to print it at 6 inches square on your desktop ink-jet printer. Because you want to have a final resolution of 200 dpi (the ideal ink-jet resolution), you should scan at 600 dpi. In an image editor, you can resize the 2-by-2-inch 600-dpi image to a 6-by-6-inch 200-dpi image with no loss of data and no interpolation.
Bring Scans into Focus
Although scanning correctly saves you a lot of trouble, there are some problems you can't avoid. For example, even the best of desktop scans are less sharp than the original photographs. In these cases, an image editor's sharpening feature can help restore what was lost. Photoshop's Unsharp Mask filter (under the Sharpen submenu in the Filter menu) is a popular choice.
Properly applied, the Unsharp Mask filter improves the appearance of a fuzzy photo. But if you oversharpen, colors alter and artifacts such as the halo around the bird's head appear.
One cautionary note: Be sure to wait until you've performed all other edits before applying sharpening.